Isoscapes of remnant and restored Hawaiian montane forests reveal differences in biological nitrogen fixation and carbon inputs

Christopher B Wall; Sean O I Swift; Carla M D'Antonio; Gerhard Gebauer; Nicole A Hynson

doi:10.7717/peerj.15468

Isoscapes of remnant and restored Hawaiian montane forests reveal differences in biological nitrogen fixation and carbon inputs

PeerJ. 2023 Jun 6:11:e15468. doi: 10.7717/peerj.15468. eCollection 2023.

Authors

Christopher B Wall^{1

2}, Sean O I Swift², Carla M D'Antonio³, Gerhard Gebauer⁴, Nicole A Hynson²

Affiliations

¹ University of California, San Diego, San Diego, CA, United States.
² University of Hawaii at Manoa, Honolulu, HI, United States.
³ University of California, Santa Barbara, Santa Barbara, CA, United States.
⁴ Universität Bayreuth, Bayreuth, Germany.

Abstract

Deforestation and subsequent land-use conversion has altered ecosystems and led to negative effects on biodiversity. To ameliorate these effects, nitrogen-fixing (N₂-fixing) trees are frequently used in the reforestation of degraded landscapes, especially in the tropics; however, their influence on ecosystem properties such as nitrogen (N) availability and carbon (C) stocks are understudied. Here, we use a 30-y old reforestation site of outplanted native N₂-fixing trees (Acacia koa) dominated by exotic grass understory, and a neighboring remnant forest dominated by A. koa canopy trees and native understory, to assess whether restoration is leading to similar N and C biogeochemical landscapes and soil and plant properties as a target remnant forest ecosystem. We measured nutrient contents and isotope values (δ¹⁵N, δ¹³C) in soils, A. koa, and non-N₂-fixing understory plants (Rubus spp.) and generated δ¹⁵N and δ¹³C isoscapes of the two forests to test for (1) different levels of biological nitrogen fixation (BNF) and its contribution to non-N₂-fixing understory plants, and (2) the influence of historic land conversion and more recent afforestation on plant and soil δ¹³C. In the plantation, A. koa densities were higher and foliar δ¹⁵N values for A. koa and Rubus spp. were lower than in the remnant forest. Foliar and soil isoscapes also showed a more homogeneous distribution of low δ¹⁵N values in the plantation and greater influence of A. koa on neighboring plants and soil, suggesting greater BNF. Foliar δ¹³C also indicated higher water use efficiency (WUE_i) in the plantation, indicative of differences in plant-water relations or soil water status between the two forest types. Plantation soil δ¹³C was higher than the remnant forest, consistent with greater contributions of exotic C₄-pasture grasses to soil C pools, possibly due to facilitation of non-native grasses by the dense A. koa canopy. These findings are consequential for forest restoration, as they contribute to the mounting evidence that outplanting N₂-fixing trees produces different biogeochemical landscapes than those observed in reference ecosystems, thereby influencing plant-soil interactions which can influence restoration outcomes.

Keywords: Acacia koa; Forests; Hakalau; Hawaii; Restoration; Stable isotopes.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Carbon
Ecosystem*
Forests
Hawaii
Nitrogen
Nitrogen Fixation
Poaceae
Rubus*
Soil
Trees
Water

Substances

Carbon
Nitrogen
Soil
Water

Grants and funding

This work was supported by the National Science Foundation (award numbers 1556856, 1557177, 2124922) and a Junior Researcher International Fellowship from the University of Bayreuth to Nicole A. Hynson. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.